The invention concerns a method for the open-loop and closed-loop control of an internal combustion engine, in which the rail pressure is controlled by closed-loop control during normal operation, and in which, when a defective rail pressure sensor is detected, the operating mode is switched from normal operating mode to emergency operating mode, in which the rail pressure is then controlled by open-loop control.
In an internal combustion engine with a common rail system, the quality of combustion is critically determined by the pressure level in the rail. Therefore, in order to stay within legally prescribed emission limits, the rail pressure is automatically controlled. A closed-loop rail pressure control system typically comprises a comparison point for determining a control deviation, a pressure controller for computing a control signal, the controlled system, and a software filter in the feedback path for computing the actual rail pressure. The control deviation is computed as the difference between a set rail pressure and the actual rail pressure. The controlled system comprises the pressure regulator, the rail, and the injectors for injecting the fuel into the combustion chambers of the internal combustion engine.
DE 10 2006 040 441 B3 describes a common rail system with closed-loop pressure control, in which the pressure controller acts on a suction throttle by means of a control signal. The suction throttle in turn determines the admission cross section to the high-pressure pump and thus the volume of fuel delivered. The suction throttle is actuated in negative logic, i.e., it is completely open at a current value of zero amperes. As a protective measure against excessively high rail pressure, for example, after a cable break in the power supply to the suction throttle, a passive pressure control valve is provided. If the rail pressure rises above a critical value, for example, 2400 bars, the pressure control valve opens. The fuel is then redirected from the rail to the fuel tank through the open pressure control valve. With the pressure control valve open, a pressure level develops in the rail which depends on the injection quantity and the engine speed. Under idling conditions, this pressure level is about 900 bars, under a full load, it is about 700 bars.
DE 101 57 641 A1 describes a common rail system, in which, when a defective rail pressure sensor is detected, a change is made from normal operating mode with closed-loop pressure control to emergency operating mode, in which the rail pressure is controlled by open-loop control. In order to avoid an undefined operating state during the transition from normal operating mode to emergency operating mode, a transition function is provided. This transition function is previously determined during normal operation from the variation of the control deviation of the rail pressure with respect to time. With the end of normal operation, a negative control deviation is then assigned to the pressure controller by the transition function. As an alternative, provision can be made to preassign a correction volume flow to the controlled system. This solution has proven effective in practice, although it has been observed that, after failure of the rail pressure sensor, the rail pressure does not always swing back to the same pressure level and therefore causes different engine outputs in emergency operating mode.